Images, whether still images or image frames of an image sequence, are generally represented on a limited number of bits (for instance 8, 10, 12 or more bits), corresponding to a limited range of values to represent the luminance signal. Images represented in such way are called images of low dynamic range or, shortly, LDR images. However the human visual system is able to perceive a wider range of luminance. The limited representation most often does not allow for reconstructing correctly small signal variations, in particular in image areas of extremely dark or bright luminance. The HDR (High Dynamic Range) format consists in significantly extending bit-depth of signal representation to integer representation with more bits e.g. 20 to 64 bits, or even to floating representation, in order to keep a high accuracy of the signal on its entire luminance range.
HDR images can be captured in various ways. For instance, Digital Single Lens Reflex cameras can use bracketing technique to capture successive images of the same scene with different exposures wherein exposure is the total density of light allowed to fall on the imaging medium (photographic film or image sensor) during the process of taking an image. Those images of different exposures are represented as LDR images. Under-exposed images capture details in the bright areas whereas over-exposed images capture details in the dark areas, as exemplarily depicted in FIG. 1 for different exposure values EV.
By fusion of these differently exposed LDR images an HDR image can be produced with a floating point representation, the produced HDR image containing all details those in dark areas as well as those in bright areas.
An HDR image cannot be used in its source format with devices designated for use with LDR images, e.g. set-top-boxes, PVR, and legacy displays. Anyway, a process called Tone Mapping, allows representing the image while ensuring a good restitution of the different signal intensity segments, in particular, in dark and bright intensity ranges. Tone Mapping creates, from an HDR image, a LDR image where all the elements are correctly exposed. The LDR image is much more detailed both in dark areas and in white areas. This is exemplarily depicted in FIG. 2.
HDR is used, in particular, in post-production. All special effects tools are dealing with HDR images with a floating point representation. The mixing being natural scene and special effects is also realized in HDR representation. At the end of the post-production process Tone Mapping is applied to create a standard 8/10/12-bit master under the control of the Director of Photography.
Mantiuk et. al.: “Backward Compatible High Dynamic Range MPEG Video Compression”, Proc. of SIGGRAPH '06 (Special issue of ACM Transactions on Graphics), 25 (3), pp. 713-723, 2006, propose a backward-compatible HDR video compression (HDR MPEG) method which introduces a compact reconstruction function that is used to decompose an HDR video stream into a residual stream and a standard LDR stream, which can be played on existing MPEG decoders, such as DVD players. Mantiuk's residual stream creates about 30% bitstream overhead compared to the LDR stream only.
Motra and Thoma: “An Adaptive LogLuv Transform for High Dynamic Range Video Compression”, Proceedings of 2010 IEEE 17th International Conference on Image Processing, September 2010, Hong Kong, describe an approach of how an existing encoder can be used for encoding HDR video sequence with an adaptive LogLuv transform proposed by the authors. Motra and Thoma further describe using the high bit depth profile of MPEG AVC to encode those data.